The field is combustion chambers for internal combustion engines. In particular, the field includes constructions for opposed-piston engines in which a combustion chamber is defined between end surfaces of pistons disposed in opposition in the bore of a ported cylinder. More particularly, the field includes opposed-piston engines with combustion chamber constructions that produce a tumbling motion in charge air admitted into the cylinder between the piston end surfaces.
Per FIG. 1, an opposed-piston engine includes at least one cylinder 10 with a bore 12 and longitudinally-displaced intake and exhaust ports 14 and 16 machined or formed therein. One or more fuel injectors 17 are secured in injector ports (ports where injectors are positioned) that open through the side surface of the cylinder. Two pistons 20, 22 according to the prior art are disposed in the bore 12 with their end surfaces 20e, 22e in opposition to each other. For convenience, the piston 20 is denominated as the “intake” piston because of its proximity to the intake port 14. Similarly, the piston 22 is denominated as the “exhaust” piston because of its proximity to the exhaust port 16.
Operation of an opposed-piston engine with one or more ported cylinders (cylinders with one or more of intake and exhaust ports formed therein) such as the cylinder 10 is well understood. In this regard, in response to combustion the opposed pistons move away from respective top dead center (TDC) positions where they are at their innermost positions in the cylinder 10. While moving from TDC, the pistons keep their associated ports closed until they approach respective bottom dead center (BDC) positions where they are at their outermost positions in the cylinder. The pistons may move in phase so that the intake and exhaust ports 14, 16 open and close in unison. Alternatively, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times.
In many opposed piston constructions, a phase offset is introduced into the piston movements. For example, presume the exhaust piston leads the intake piston and the phase offset causes the pistons to move around their BDC positions in a sequence in which the exhaust port 16 opens as the exhaust piston 22 moves through BDC while the intake port 14 is still closed so that combustion gasses start to flow out of the exhaust port 16. As the pistons continue moving away from each other, the intake piston 20 moves through BDC causing the intake port 14 to open while the exhaust port 16 is still open. A charge of pressurized air is forced into the cylinder 10 through the open intake port 14, driving exhaust gasses out of the cylinder through the exhaust port 16. As seen in FIG. 1, after further movement of the pistons, the exhaust port 16 closes before the intake port 14 while the intake piston 20 continues to move away from BDC. Typically, the charge of fresh air is swirled as it passes through ramped openings of the intake port 14. With reference to FIG. 1, the swirling motion (or simply, “swirl”) is a generally helical movement of charge air that circulates around the cylinder's longitudinal axis and moves longitudinally through the bore of the cylinder 10. Per FIG. 2, as the pistons 20, 22 continue moving toward TDC, the intake port 14 is closed and the swirling charge air remaining in the cylinder is compressed between the end surfaces 20e and 22e. As the pistons near their respective TDC locations in the cylinder bore, fuel 40 is injected into the compressed charge air 30, between the end surfaces 20e, 22e of the pistons. As injection continues, the swirling mixture of air and fuel is increasingly compressed in a combustion chamber 32 defined between the end surfaces 20e and 22e as the pistons 20 and 22 move through their respective TDC locations. When the mixture reaches an ignition temperature, the fuel ignites in the combustion chamber, driving the pistons apart toward their respective BDC locations.
The geometries of the intake port openings and the cylinder of an opposed-piston engine provide a very effective platform for generation of a strong bulk fluid motion of the charge air in the form of swirl that promotes both removal of exhaust gasses (scavenging) and the movement of fuel to air (air/fuel mixing). However, charge air motion that is dominated by swirl can produce undesirable effects during combustion. For example, during combustion in a cylindrical combustion chamber defined between flat piston end surfaces, swirl pushes the flame toward the cylinder bore, causing heat loss to the (relatively) cooler cylinder wall. The higher velocity vectors of swirl occur near the cylinder wall, which provides the worst scenario for heat losses: high temperature gas with velocity that transfers heat to the cylinder wall and lowers the thermal efficiency of the engine. The peripheries of the piston end surfaces also receive a relatively high heat load, which causes formation of a solid residue of oil coke that remains in the piston/cylinder interface when lubricating oil breaks down at high engine temperatures. Accordingly, in such opposed-piston engines, it is desirable to maintain the swirl of charge air as injection starts while mitigating its undesirable effects as combustion begins.
As fuel injection begins, it is desirable to generate turbulence in the charge air motion in order to encourage a more homogeneous mixture of fuel and air, which in turn, produces more complete and more uniform ignition than would otherwise occur.
In certain opposed-piston combustion chamber constructions, charge air turbulence is produced by squish flow from the periphery of the combustion chamber in a radial direction of the cylinder toward the cylinder's axis. Squish flow is generated by movement of compressed air from a relatively high-pressure region at the peripheries of the piston end surfaces to a lower-pressure region generated by a bowl formed in at least one piston end surface. Squish flow promotes charge air turbulence in the combustion chamber. For example, U.S. Pat. No. 6,170,443 discloses a cylinder with a pair of opposed pistons having complementary end surface constructions. A circular concave depression formed in one end surface is symmetrical with respect to the axis of its piston and rises to a point in its center. The periphery of the opposing end surface has a convex shape in the center of which a semi-toroidal (half donut-shaped) trench is formed. As the pistons approach TDC, they define a generally toroidally-shaped combustion chamber centered on the longitudinal axis of the cylinder. The combustion chamber is surrounded by a circumferential squish band defined between the concave and convex surface shapes. As the pistons approach TDC, the squish band generates an inwardly-directed squish flow into the toroidal trench and creates “a swirl of high intensity near top dead center.” See the '443 patent at column 19, lines 25-27. Fuel is injected into the toroidal combustion chamber in a radial direction of the bore.
It is desirable to increase the turbulence of charge air in the combustion chamber so as to produce a more uniform mixture of air and fuel. Domination of charge air motion by swirl or squish flow alone does achieve a certain level of turbulence. Nevertheless, it is desirable to create additional elements of charge air motion as injection commences in order to produce even more turbulence of the charge air, thereby to achieve better air/fuel mixing than can be obtained with swirl or squish alone.